Introduction

There are times when a generic (in the sense of
general as opposed to
template-based programming) type is needed:
variables that are truly variable, accommodating values of many
other more specific types rather than C++'s normal strict and
static types. We can distinguish three basic kinds of generic
type:

Converting types that can hold one of a number of
possible value types, e.g. int and
string, and freely convert between them, for
instance interpreting 5 as "5" or
vice-versa. Such types are common in scripting and other
interpreted
languages.
boost::lexical_cast
supports such conversion functionality.

Discriminated types that contain values of different types but
do not attempt conversion between them, i.e. 5 is
held strictly as an int and is not implicitly
convertible either to "5" or to
5.0. Their indifference to interpretation but
awareness of type effectively makes them safe, generic
containers of single values, with no scope for surprises from
ambiguous conversions.

Indiscriminate types that can refer to anything but are
oblivious to the actual underlying type, entrusting all forms
of access and interpretation to the programmer. This niche is
dominated by void *, which offers plenty of scope
for surprising, undefined behavior.

The boost::any class
(based on the class of the same name described in "Valued
Conversions" by Kevlin Henney, C++
Report 12(7), July/August 2000) is a variant value type
based on the second category. It supports copying of any value
type and safe checked extraction of that value strictly against
its type. A similar design, offering more appropriate operators,
can be used for a generalized function adaptor,
any_function, a generalized iterator adaptor,
any_iterator, and other object types that need
uniform runtime treatment but support only compile-time template
parameter conformance.